Method of pre-processing over-write capable magnetooptical recording medium, and pre-processed over-write capable magnetooptical recording medium

ABSTRACT

PCT No. PCT/JP90/01396 Sec. 371 Date Jun. 26, 1991 Sec. 102(e) Date Jun. 26, 1991 PCT Filed Oct. 31, 1990 PCT Pub. No. WO91/06951 PCT Pub. Date May 16, 1991.An over-write capable magnetooptical recording medium is pre-processed by subjecting a track separation zone to a pre-processing field which aligns the magnetization of a recording layer in a predetermined direction so as to avoid formation, in the separation zone, of a magnetic wall between the recording layer and a reference layer when the reference layer is subjected to an initial field to align its direction of magnetization.

TECHNICAL FIELD

The present invention relates to a method of pre-processing amagnetooptical recording medium disk capable of performing an over-writeoperation by only intensity modulation of light without modulating adirection of a bias field, and the pre-processed disk.

BACKGROUND ART

In recent years, many efforts have been made to develop an opticalrecording/reproduction method, apparatus and medium which can satisfyvarious requirements including high density, large capacity, high accessspeed, and high recording/reproduction speed.

Of various optical recording/reproduction methods, the magnetoopticalrecording/reproduction method is most attractive due to its uniqueadvantages in that information can be erased after it is used, and newinformation can be recorded.

A recording medium used in the magnetooptical recording/reproductionmethod has a perpendicular magnetic anisotropy layer or layers as arecording layer. The magnetic layer comprises, for example, amorphousGdFe GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentrical orspiral tracks are normally formed on the recording layer, andinformation is recorded on the tracks. In this specification, one of the"upward" and "downward" directions of the magnetization with respect toa film surface is defined as an "A direction", and the other one isdefined as a "non-A direction". Information to be recorded is binarizedin advance, and is recorded by two signals, i.e., a bit (B₁) having an"A-directed" magnetization, and a bit (B₀) having a "non-A-directed"magnetization. These bits B₁ and B₀ correspond to "1" and "0" levels ofa digital signal. The direction of magnetization of the recording trackscan be aligned in the "non-A direction" by applying a strong bias field.This processing is called "initialization". Thereafter, a bit (B₁)having an "A-directed" magnetization is formed on the tracks.

The principle of bit formation will be described below with reference toFIG. 1.

In the bit formation, a characteristic feature of laser, i.e., excellentcoherence in space and time, is effectively used to focus a beam into aspot as small as the diffraction limit determined by the wavelength ofthe laser light. The focused light is radiated onto the track surface towrite information by producing bits less than 1 μm in diameter on therecording layer. In the optical recording, a recording density up to 10⁸bit/cm² can be theoretically attained, since a laser beam can beconcentrated into a spot with a size as small as its wavelength.

As shown in FIG. 1, in the magnetooptical recording, a laser beam L isfocused onto a recording layer 1 to heat it, while a bias field Hb isexternally applied to the heated portion in the direction opposite tothe initialized direction. A coercivity (denoted Hc herein) of thelocally heated portion is decreased below the bias field Hb. As aresult, the direction of magnetization of that portion is aligned in thedirection of the bias field Hb. In this way, reversely magnetized bitsare formed.

Ferromagnetic and ferrimagnetic materials differ in the temperaturedependencies of the magnetization and Hc. Ferromagnetic materials haveHc which decreases around the Curie temperature and allow informationrecording based on this phenomenon. Thus, information recording inferromagnetic materials is referred to as Tc recording (Curietemperature recording).

On the other hand, ferrimagnetic materials have a compensationtemperature, below the Curie temperature, at which magnetization Mbecomes zero. The Hc abruptly increases around this temperature andhence abruptly decreases outside this temperature. The decreased Hc iscanceled by a relatively weak bias field Hb. Namely, recording isenabled. This process is called T_(comp). recording (compensation pointrecording).

In this case, however, there is no need to adhere to the Curie point ortemperatures therearound, and the compensation temperature In otherwords, if a bias field Hb capable of canceling a decreased Hc is appliedto a magnetic material having the decreased Hc at a predeterminedtemperature higher than a room temperature, recording is enabled.

The principle of reproduction will be described below with reference toFIG. 2.

FIG. 2 illustrates the principle of information reproduction based onthe magnetooptical effect. Light is an electromagnetic wave with anelectromagnetic-field vector normally emanating in all directions in aplane perpendicular to the light path. When light is converted tolinearly polarized beams L_(P) and radiated onto a recording layer 1, itis reflected by the surface or passes through the recording layer 1. Atthis time, the plane of polarization rotates according to the directionof magnetization (M). This phenomenon is called the magnetic Kerr effector magnetic Faraday effect.

For example, if the plane of polarization of the reflected light rotatesthrough θ_(k) degrees for the "A-directed" magnetization, it rotatesthrough -θ_(k) degrees for the "non-A-directed" magnetization.Therefore, when the axis of an optical analyzer (polarizer) is setperpendicular to the plane inclined at -θ_(k), the light reflected by"non-A-direction" magnetized bit B₀ cannot pass through the analyzer. Onthe contrary, a product of (sin 2θ_(k))² and the light reflected by abit B₁ magnetized along the "A direction" passes through the analyzerand becomes incident on a detector (photoelectric conversion means). Asa result, the bit B₁ magnetized along the "A direction" looks brighterthan the bit B₀ magnetized along the "non-A direction", and the detectorproduces a stronger electrical signal for the bit B₁. The electricalsignal from the detector is modulated in accordance with the recordedinformation, thus reproducing the information.

In order to re-use a recorded medium, (i) the medium must bere-initialized by an initializing device, or (ii) an erase head havingthe same arrangement as a recording head must be added to a recordingapparatus, or (iii) as preliminary processing, recorded information mustbe erased using a recording apparatus or an erasing apparatus.Therefore, in the conventional magnetooptical recording method, it isimpossible to perform an over-write operation, which can properly recordnew information regardless of the presence/absence of recordedinformation.

If the direction of a bias field Hb can be desirably modulated betweenthe "A-direction" and "non-A direction", an over-write operation ispossible. However, it is impossible to modulate the bias field Hb athigh speed. For example, if the bias field Hb comprises a permanentmagnet, the direction of the magnet must be mechanically reversed.However, it is impossible to reverse the direction of the magnet at highspeed. Even when the bias field Hb comprises an electromagnet, it isalso impossible to modulate the direction of a large-capacity current athigh speed.

However, according to remarkable technical developments, amagnetooptical recording method capable of performing an over-writeoperation by modulating only an intensity of light to be radiated inaccordance with binary information to be recorded without turning on/offthe bias field Hb or without modulating the direction of the bias fieldHb, an over-write capable magnetooptical recording medium used in thismethod, and an over-write capable recording apparatus used in thismethod were invented and filed as a patent application (U.S. Ser. No.453,255 filed on Dec. 20, 1989). The basic invention disclosed in theabove-mentioned patent application will be described below.

One of the characteristic features of the basic invention is to use amagnetooptical recording medium comprising a multilayered perpendicularmagnetic anisotropy film having at least a two-layered structureincluding a recording layer (first layer) and a reference layer (secondlayer). Information is recorded in the first layer (in some cases, alsoin the second layer) by a bit having an "A-directed" magnetization, anda bit having a "non-A-directed" magnetization.

An over-write method according to the basic invention comprises:

(a) moving a recording medium;

(b) applying an initial field Hini. to align a direction ofmagnetization of only the second layer in the "A direction" while thedirection of magnetization of the first layer is left unchanged beforerecording;

(c) radiating a laser beam on the medium;

(d) pulse-modulating the beam intensity i accordance with binaryinformation to be recorded;

(e) applying a bias field to the radiated portion when the beam isradiated; and

(f) forming one of a bit having the "A-directed" magnetization and a bithaving the "non-A-directed" magnetization when the intensity of thepulse-modulated beam is at high level, and forming the other bit whenthe beam intensity is at low level.

When recording is performed, the basic invention employs, for example,an over-write capable magnetooptical recording apparatus comprising:

(a) means for moving a magnetooptical recording medium;

(b) initial field Hini. apply means;

(c) a laser beam source;

(d) means for pulse-modulating a beam intensity in accordance withbinary information to be recorded to obtain high level that provides, tothe medium, a temperature suitable for forming one of a bit having an"A-directed" magnetization and a bit having a "non-A-directed"magnetization, and to obtain low level that provides, to the medium, atemperature suitable for forming the other bit; and

(e) bias field apply means which can be commonly used as the initialfield apply means.

In the basic invention, a laser beam is pulse-modulated according toinformation to be recorded. This procedure itself has been performed inthe conventional magnetooptical recording method, and a means forpulse-modulating the beam intensity on the basis of binary informationto be recorded is a known means. For example, see "THE BELL SYSTEMTECHNICAL JOURNAL, Vol. 62 (1983), pp. 1923-1936 for further details.Therefore, the modulating means is available by partially modifying theconventional beam modulating means if required high and low levels ofthe beam intensity are given. Such a modification would be easy forthose who are skilled in the art if high and low levels of the beamintensity are given

Another characteristic feature of the basic invention lies in high andlow levels of the beam intensity. More specifically, when the beamintensity is at high level, "A-directed" magnetization of a referencelayer (second layer) is reversed to the "non-A direction" by means of abias field (Hb), and a bit having the "non-A-directed" [or "A-directed"]magnetization is thus formed in a recording layer (first layer) by meansof the "non-A-directed" magnetization of the second layer. When the beamintensity is at low level, a bit having the "A-directed" [or"non-A-directed"] magnetization is formed in the first layer by means ofthe "A-directed" magnetization of the second layer.

In this specification, if expressions ooo [or ΔΔΔ] appear, ooo outsidethe parentheses in the first expression corresponds to ooo in thesubsequent expressions ooo [or ΔΔΔ], and vice versa.

As is well known, even if recording is not performed, a laser beam isoften turned on at first low level in order to, for example, access apredetermined recording position on the medium. When the laser beam isalso used for reproduction, the laser beam is often turned on at anintensity of the first low level. In this invention, the intensity ofthe laser beam may be set at this first low level. However, level forforming a bit is second low level higher than the first low level.Therefore, the output waveform of the laser beam of the basic inventionis as shown in FIG. 3.

Although not described in the specification of the basic invention, arecording beam need not always be a single beam but may be two proximitybeams in the basic invention. More specifically, a first beam may beused as a low-level laser beam (erasing beam) which is not modulated inprinciple, and a second beam may be used as a high-level laser beam(writing beam) which is modulated in accordance with information. Inthis case, the second beam is pulse-modulated between high level andbase level (equal to or lower than low level, and its output may bezero). In this case, an output waveform is as shown in FIG. 4.

A medium used in the basic invention is roughly classified into a firstor second category. In either category, a recording medium has amultilayered structure including a recording layer (first layer) and areference layer (second layer), as shown in FIG. 5.

The first layer is the recording layer, which exhibits a high coercivityat a room temperature, and has a low magnetization reversingtemperature. The second layer is the reference layer, which exhibits arelatively lower coercivity at a room temperature and has a highermagnetization reversing temperature than those of the first layer. Eachof the first and second layers may comprise a multilayered structure Ifnecessary, a third layer may be interposed between the first and secondlayers. In addition, a clear boundary between the first and secondlayers need not be formed, and one layer can be gradually converted intothe other layer.

In the first category, when the coercivity of the recording layer isrepresented by H_(c1) ; that of the reference layer, H_(c2) ; a Curietemperature of the first layer, T_(c1) ; that of the second layer,T_(c2) ; a room temperature, T_(R) ; a temperature of the recordingmedium obtained when a low-level laser beam is radiated, T_(L) ; thatobtained when a high-level laser beam is radiated, T_(H) ; a couplingfield applied to the first layer, H_(D1) ; and a coupling field appliedto the second layer, H_(D2), the recording medium satisfies Formula 1below, and satisfies Formulas 2 to 5 at the room temperature:

    T.sub.R <T.sub.c1 ≈T.sub.L <T.sub.c2 ≈T.sub.H 1

    H.sub.c1 >H.sub.c2 +|H.sub.D1 ∓H.sub.D2 |2

    H.sub.c1 >H.sub.D1                                         3

    H.sub.c2 >H.sub.D2                                         4

    H.sub.c2 +H.sub.D2 <|Hini.|<H.sub.c1 ±H.sub.D1 5

In the above formulas, symbol "≈" means "equal to" or "substantiallyequal to". In addition, of double signs ± and ∓, the upper signcorresponds to an A (antiparallel) type medium, and the lower signcorresponds to a P (parallel) type medium (these media will be describedlater). Note that a ferromagnetic medium belongs to a P type.

The relationship between a coercivity and a temperature is shown in FIG.6. A first curve represents the characteristics of the first layer, anda second curve represents those of the second layer.

Therefore, when an initial field (Hini.) is applied to this recordingmedium at the room temperature, the direction of magnetization of onlythe reference layer (second layer) is reversed without reversing that ofthe recording layer (first layer) according to Formula 5. When theinitial field (Hini.) is applied to the medium before recording, onlythe second layer can be magnetized in the "A direction" (in thedrawings, the "A direction" is indicated by an upward arrow , and the"non-A direction" is indicated by a downward arrow for the sake ofsimplicity). If the initial field Hini. becomes zero, the direction ofmagnetization of the second layer can be left unchanged without beingre-reversed according to Formula 4.

FIG. 7 schematically shows a state wherein only the second layer ismagnetized in the "A direction" immediately before recording. Thedirection of magnetization * in the first layer represents previouslyrecorded information. FIG. 8 illustrates a direction of magnetizationwhen a high-level laser beam is radiated on the medium shown in FIG. 7,and a bit whose direction of magnetization of the first layer can bedisregarded is indicated by X.

In a first condition, a high-level laser beam is radiated to increase amedium temperature to T_(H). Since T_(H) is higher than the Curietemperature T_(c1), the magnetization of the recording layer (firstlayer) disappears. In addition, since T_(H) is near the Curietemperature T_(c2), the magnetization of the reference layer (secondlayer) also disappears completely or almost completely. The bias field(Hb) in the "A direction" or "non-A direction" is applied to the mediumin accordance with the type of medium. The bias field (Hb) can be astray field from the medium itself. For the sake of simplicity, assumethat the bias field (Hb) in the "non-A direction" is applied to themedium. Since the medium is moving, a given irradiated portion isimmediately separated apart from the laser beam, and is cooled. When themedium temperature is decreased under the presence of Hb, the directionof magnetization of the second layer is reversed to the "non-Adirection" based on Hb (Condition 2_(H)).

When the medium is further cooled and the medium temperature isdecreased slightly below T_(c1), Condition 3_(H) is established, andmagnetization of the first layer appears again. In this case, thedirection of magnetization of the first layer is influenced by that ofthe second layer due to a magnetic coupling (exchange coupling) force.As a result, magnetization (the P type medium) or (the A type medium) isformed according to the type of medium.

A change in condition caused by the high-level laser beam will be calleda high-temperature cycle herein.

Referring to FIG. 9, a low-level laser beam is radiated to increase themedium temperature to T_(L), thus establishing Condition 2_(L). SinceT_(L) is near the Curie temperature T_(c1), the magnetization of thefirst layer disappears completely or almost completely. However, sinceT_(L) is lower than the Curie temperature T_(c2), the magnetization ofthe second layer does not disappear.

Although the bias field (Hb) is unnecessary, it cannot be turned on oroff at high speed (within a short period of time). Therefore, the biasfield (Hb) in the high-temperature cycle is left on.

However, since the H_(c2) is kept high, the magnetization of the secondlayer will not be reversed by Hb. Since the medium is moving, a givenirradiated portion is immediately separated apart from the laser beam,and is cooled. As cooling progresses, Condition 3_(L) is established,and the magnetization of the first layer appears again. The direction ofmagnetization appearing in this case is influenced by that of the secondlayer due to the magnetic coupling force. As a result, (P type) or (Atype) magnetization appears according to the type of medium.

A change in condition caused by the low-level laser beam will be calleda low-temperature cycle herein.

As described above, bits having either magnetization or , which areopposite to each other, are formed in the high- and low-temperaturecycles regardless of the direction of magnetization of the first layer.More specifically, an over-write operation is enabled bypulse-modulating the laser beam between high level (high-temperaturecycle) and low level (low-temperature cycle) in accordance withinformation to be recorded. This is represented in FIG. 10.

In the above description, both the first and second layers have nocompensation temperature T_(comp). between the room temperature and theCurie temperature. However, when the compensation temperature T_(comp).is present, if the medium temperature exceeds it, the direction ofmagnetization is reversed, and a change in direction differs dependingon A and P types. In addition, the direction of the bias field Hb isopposite to the direction ↓ in the above description at the roomtemperature

A recording medium normally has a disk shape, and is rotated duringrecording. For this reason, a recorded portion (bit) is influenced againby the initial field Hini. during one revolution. As a result, thedirection of magnetization of the reference layer (second layer) isaligned in the original "A direction" . However, at the roomtemperature, the magnetization of the second layer can no longerinfluence that of the recording layer (first layer), and the recordedinformation can be held.

If linearly polarized light is radiated on the first layer, since lightreflected thereby includes information, the information can bereproduced as in the conventional magnetooptical recording medium. Inaddition, a method of transferring information in the first layer to thesecond layer aligned in the original "A direction" by applying areproduction field H_(R) before reproduction or a method of naturallytransferring information in the first layer to the second layer as soonas the influence of Hini. disappears without applying the reproductionfield H_(R) is also available depending on composition designs of thefirst and second layers. In this case, information may be reproducedfrom the second layer.

A perpendicular magnetic anistotroy film constituting each of therecording layer (first layer) and the reference layer (second layer) isselected from the group consisting of ferromagnetic and ferrimagneticmaterials having no compensation temperature and having a Curietemperature, and an amorphous or crystalline ferrimagnetic materialhaving both the compensation temperature and the Curie temperature.

The first category which utilizes the Curie temperature as themagnetization reversing temperature has been described. In contrast tothis, the second category utilizes decreased H_(c) at a predeterminedtemperature higher than the room temperature. In a medium of the secondcategory, substantially the same description as the first category canbe applied except that a temperature T_(s1) at which the recording layer(first layer) is magnetically coupled to the reference layer (secondlayer) is used in place of T_(c1) in the first category, and atemperature T_(s1) at which the direction of magnetization of the secondlayer is reversed by Hb is used in place of T_(c2).

In the second category, when the coercivity of the first layer isrepresented by H_(c1) ; that of the second layer, H_(c2) ; a temperatureat which the first layer is magnetically coupled to the second layer,T_(s1) ; a temperature at which the magnetization of the second layer isreversed by Hb, T_(s2) ; a room temperature, T_(R) ; a mediumtemperature obtained when a low-level laser beam is radiated, T_(L) ;that obtained when a high-level laser beam is radiated, T_(H) ; acoupling field applied to the first layer, H_(D1) ; and a coupling fieldapplied to the second layer, H_(D2), the recording medium satisfiesFormula 6 below, and satisfies Formulas 7 to 10 at the room temperature:

    T.sub.R <T.sub.s1 ≈T.sub.L<T.sub.s2 ≈T.sub.H 6

    H.sub.c1 >H.sub.c2 +|H.sub.D1 ∓H.sub.D2 |7

    H.sub.c1 >H.sub.D1                                         8

    H.sub.c2 >H.sub.D2                                         9

    H.sub.c2 +H.sub.D2 <|Hini.|<H.sub.c1 ±H.sub.D1 10

In the above formulas, of double signs ± and ∓, the upper signcorresponds to an A type medium, and the lower sign corresponds to a Ptype medium (these media will be described later).

Referring to FIG. 11, in the second category, when the medium is at thehigh temperature T_(H), the magnetization of the second layer does notdisappear, but is sufficiently weak. The magnetization of the firstlayer disappears, or is sufficiently weak. Even if sufficiently weakmagnetization is left in both the first and second layers, the biasfield Hb is sufficiently large, and forces the direction ofmagnetization of the second layer and that of the first layer in somecases to follow that of the Hb, thus establishing Condition 2_(H).

Thereafter, the second layer influences the first layer via σ_(w)immediately, or when cooling progresses after radiation of the laserbeam is stopped and the medium temperature is decreased below T_(H), orwhen the irradiated portion is away from Hb, thereby aligning thedirection of magnetization of the first layer in a stable direction. Asa result, Condition 3_(H) is established. When the magnetization of thefirst layer originally has a stable direction, it is left unchanged.

Referring to FIG. 12, when the medium is at the low temperature T_(L),both the first and second layers do not lose their magnetization.However, the magnetization of the first layer is sufficiently weak.Therefore, the direction of magnetization of the first layer isinfluenced, via σ_(w), by the magnetization of the second layer which ismore largely influenced by Hb. In this case, since the second layer hassufficient magnetization, its magnetization will not be reversed by Hb.As a result, Condition 3_(L) is established regardless of Hb.

In the above description, both the first and second layers have nocompensation temperature T_(comp). between the room temperature and theCurie temperature. When the compensation temperature T_(comp). ispresent, a more complex situation obtains as described above, and thedirection of the bias field is opposite to the direction at the roomtemperature. In both the first and second categories, the recordingmedium is preferably constituted by the recording layer (first layer)and the reference layer (second layer) each of which comprises anamorphous ferrimagnetic material selected from transition metal (e.g.,Fe, Co)--heavy rare earth metal (e.g., Gd, Tb, Dy, and the like) alloycompositions.

When the materials of both the first and second layers are selected fromthe transition metal-heavy rare earth metal alloy compositions, thedirection and level of magnetization appearing outside the alloys aredetermined by the relationship between the direction and level of spinof transition metal (to be abbreviated to as TM hereinafter) atoms, andthose of heavy rare earth metal (to be abbreviated to as RE hereinafter)atoms inside the alloys. For example, the direction and level of TM spinare represented by a dotted vector , those of RE spin are represented bya solid vector ↑, and the direction and level of magnetization of theentire alloy are represented by a double-solid vector . In this case,the vector is expressed as a sum of the vectors and ↑. However, in thealloy, the vectors and ↑ are directed in the opposite directions due tothe mutual effect of the TM spin and the RE spin. Therefore, whenstrengths of these vectors are equal to each other, the sum of and ↑ orthe sum of ↓ and is zero (i.e., the level of magnetization appearingoutside the alloy becomes zero). The alloy composition making the sum ofvectors zero is called a compensation composition. When the alloy hasanother composition, it has a strength equal to a difference between thestrengths of the two spins, and has a vector ( or ) having a directionequal to that of the larger vector. Magnetization of this vector appearsoutside the alloy. For example, appears as , and and appears as .

When one of the strengths of the vectors of the RE and TM spins islarger than the other, the alloy composition is referred to as "oo rich"named after the larger spin name (e.g., RE rich).

Both the first and second layers can be classified into TM rich and RErich compositions. Therefore, when the composition of the first layer isplotted along the ordinate and that of the second layer is plotted alongthe abscissa, the types of media as a whole of the basic invention canbe classified into four quadrants, as shown in FIG. 13. The P typemedium described above belongs to Quadrants I and III, and the A typemedium belongs to Quadrants II and IV. Note that the intersection of theabscissa and the ordinate represents the compensation composition of thetwo layers.

In view of a change in coercivity against a change in temperature, agiven alloy composition has characteristics wherein the coercivitytemporarily increases infinitely and then abruptly decreases before atemperature reaches the Curie temperature (at which the coercivity iszero) The temperature corresponding to the infinite coercivity is calleda compensation temperature (T_(comp).). No compensation temperature ispresent between the room temperature and the Curie temperature in the TMrich alloy composition. A compensation temperature below the roomtemperature is irrelevant in the magnetooptical recording, and hence, itis assumed in this specification that the compensation temperature ispresent between the room temperature and the Curie temperature.

If the first and second layers are classified in view of thepresence/absence of the compensation temperature, the recording mediumcan be classified into four types. A medium in Quadrant I includes allthe four types of media. FIGS. 14A, 14B, 14C, and 14D show therelationship between the coercivity and temperature of these four typesof media.

When the recording layer (first layer) and the reference layer (secondlayer) are classified in view of their RE or TM rich characteristics andin view of the presence/absence of the compensation temperature,recording media can be classified into the following nine classes.

                  TABLE 1                                                         ______________________________________                                        Class                         Type                                            ______________________________________                                               Quadrant I (P type)                                                             First Layer: Second Layer:                                                    RE Rich      RE Rich                                                 ______________________________________                                        1        T.sub.comp.  T.sub.comp. 1                                           2        No T.sub.comp.                                                                             T.sub.comp. 2                                           3        T.sub.comp.  No T.sub.comp.                                                                            3                                           4        No T.sub.comp.                                                                             No T.sub.comp.                                                                            4                                           ______________________________________                                               Quadrant II (A type)                                                            First Layer: Second Layer:                                                    RE Rich      TM Rich                                                 ______________________________________                                        5        T.sub.comp.  No T.sub.comp.                                                                            3                                           6        No T.sub.comp.                                                                             No T.sub.comp.                                                                            4                                           ______________________________________                                               Quadrant III (P type)                                                           First Layer: Second Layer:                                                    TM Rich      TM Rich                                                 ______________________________________                                        7        No T.sub.Comp.                                                                             No T.sub.comp.                                                                            4                                           ______________________________________                                               Quandrant IV (A type)                                                           First Layer: Second Layer:                                                    TM Rich      RE Rich                                                 ______________________________________                                        8        No T.sub.comp.                                                                             T.sub.comp. 2                                           9        No T.sub.comp.                                                                             No T.sub.comp.                                                                            4                                           ______________________________________                                    

In general, as considered from a direction perpendicular to a magneticlayer plane, spiral or concentrical tracks for recording information areformed on a disk, and a separation zone is present between adjacenttracks.

In the manufacture of a medium, directions of magnetization of magneticlayer portions located in the separation zones are often nonuniform.When over-write recording is carried out, since it is generallyimpossible to focus a magnetic field onto a narrow region as small as atrack width, an initial field Hini. is applied to the separation zoneslocated on two sides of each track, and directions of magnetization ofthe reference layer portions in the separation zones are aligned alongthe direction of the initial field Hini. Thus, in a portion where thedirection of magnetization of the recording layer in the separation zoneis unstable with respect to the reference layer, a magnetic wall isformed between the recording layer and the reference layer. If theinitial field Hini. is carelessly applied even though the directions ofmagnetization are not nonuniform, a magnetic wall may be formed in theentire region of the separation zone.

When recording is performed according to the basic invention, a C/Nratio may be decreased or previous information may be reproduced due tothe above-mentioned causes, and an information bit error rate may beundesirably increased.

The present inventors have made extensive studies, and found that when amagnetic wall is present between the recording layer and the referencelayer in the separation zone, the above-mentioned problems are posed.

It is an object of the present invention to provide a method ofprocessing a medium so as to decrease an information bit error rateduring reproduction of information.

DISCLOSURE OF INVENTION

According to the present invention, in order to achieve the aboveobject, a recording medium is used which includes a recording layer anda reference layer having a perpendicular magnetic anisotropy, and onwhich a plurality of tracks for recording information are formed andseparation zones are formed between adjacent tracks. Before informationis recorded on the medium, an initial field is applied to the medium toalign the direction of magnetization of the reference layer in a firstpredetermined direction without changing the direction of magnetizationof the recording layer. A pre-processing field is applied to theseparation zones so as to avoid the formation of a magnetic wall betweenthe recording layer and the reference layer in each separation zone uponapplication of the initial field, by aligning the direction ofmagnetization of the recording layer in each separation zone in a secondpredetermined direction.

Information is then recorded on the disk medium which is pre-processedin this manner. Before recording, the reference layer of each track ofthe medium is subjected to the initial field Hini. Since it is difficultto focus the initial field Hini. to a narrow range as small as a trackwidth, it also influences the separation zones beyond the track. Forthis reason, when an expected application direction of the initial fieldHini. is different from the direction of the initial field Hini. whichis applied in practical recording, a magnetic wall is undesirably formedbetween the recording layer and the reference layer in each separationzone.

Therefore, according to the present invention, the direction of theinitial field Hini. is indicated on a disk itself or a container storingthe disk, so that a direction of the initial field Hini. expected inpre-processing is not different from that of the initial field Hini.applied during actual recording.

According to the present invention, when the direction of magnetizationof the recording layer is aligned in a stable direction with respect tothe direction of magnetization of the reference layer in each separationzone so as to completely eliminate a magnetic wall, the followingmethods are available.

(1) A large pre-processing field is applied to the entire medium or theentire recording region where tracks are formed at normal temperature.With this method, the direction of magnetization of the recording layerin both the separation zone and track is aligned in one direction.

(2) A method of heating the entire medium or the entire recording regionwhere tracks are formed to decrease a coercivity of the medium, and thenapplying a pre-processing field to the heated portion. With this method,the direction of magnetization of the recording layer in both theseparation zone and track can also be aligned in one direction.

(3) A method of heating separation zones while applying, to theseparation zones, a pre-processing field having a strength equal to orhigher than that of a bias field Hb of the basic invention and having adirection opposite to that of the bias field Hb. As a heating method,for example, a non-modulated laser beam fixed at high level or higher isradiated on the separation zones simultaneously with application of thepre-processing field. With this method, the direction of magnetizationof the recording layer in each separation zone is aligned in onedirection.

In general, a groove is continuously or intermittently formed in eachseparation zone, and in this case, a track is called a land.

Media of the basic invention can be roughly classified into P type mediaand A type media. In the former type, when the directions ofmagnetization of the recording layer and the reference layer are equalto each other, magnetization is stable, and no magnetic wall can beformed between the two layers. In the latter type, when the directionsof magnetization of the recording layer and the reference layer areopposite to each other, magnetization is stable, and no magnetic wallcan be formed between the two layers.

The principle of an over-write operation will be described in detailbelow using a medium No. 1 belonging to Class 1 (P type, Quadrant I,Type 1) shown in Table 1.

The medium No. 1 satisfies Formula 11:

    T.sub.R <T.sub.comp.1 <T.sub.L <T.sub.H ≲T.sub.c1 ≲T.sub.c2

and also satisfies Formula 11-2:

    T.sub.comp.2 <T.sub.c1

For the sake of simplicity, a medium having a relation of T_(H) <T_(c1)<T_(c2) will be considered below. The temperature T_(comp).2 may behigher than, equal to, or lower than T_(L). For the sake of simplicity,T_(L) <T_(comp).2 in the following description. FIG. 15 shows theabove-mentioned relationship.

A condition that reverses only the direction of magnetization of thesecond layer without reversing that of the first layer (recording layer)by the initial field Hini. at the room temperature T_(R) is representedby Formula 12. This medium No. 1 satisfies Formula 12. ##EQU1## where

H_(C1) : coercivity of first layer

H_(C2) : coercivity of second layer

M_(S1) : saturation magnetization of first layer

M_(S2) : saturation magnetization of second layer

t₁ : film thickness of first layer

t₂ : film thickness of second layer

σ_(w) : interface wall energy

At this time, a condition for Hini. is represented by Formula 15. IfHini. disappears, the directions of magnetization of the first andsecond layers are influenced by each other due to the interface wallenergy. The conditions that can hold the directions of magnetization ofthe first and second layers without reversing them are represented byFormulas 13 and 14. The medium No. 1 satisfies Formulas 13 and 14.##EQU2##

The magnetization of the second layer of the recording medium whichsatisfies conditions given by Formulas 12 to 14 at the room temperatureis aligned in, e.g., the "A direction" ( ) by Hini. which satisfiesfollowing Formula 15 immediately before recording: ##EQU3## At thistime, the first layer is left in the previous recorded state, as shownin Conditions 1_(a) and 1_(b) in FIG. 16. When the direction ofmagnetization of the first layer is aligned in the "non-A direction", amagnetic wall is formed between the first and second layers. Conditions1_(a) and 1_(b) are held immediately before recording.

The bias field Hb is then applied in the "A direction" ↑.

Note that it is difficult to focus the bias field Hb to the same rangeas a radiation region (spot region) of the laser beam as well as normalmagnetic fields. When a medium has a disk shape, recorded information(bit) is influenced by Hini. during one revolution, and Condition 1appears again. The bit passes a portion near the laser beam radiationregion (spot region). At this time, the bit in Condition 1 is influencedby a bias field Hb apply means since the bit approaches it. In thiscase, if the direction of magnetization of the bit of the first layerhaving the direction of magnetization opposite to that of Hb is reversedby Hb, information which has been recorded one revolution before islost. A condition for preventing this is given by: ##EQU4## Thedisk-like medium must satisfy this formula at the room temperature. Inother words, a condition for determining Hb is expressed by Formula15-2.

The bit in Conditions 1_(a) and 1_(b) then reaches the spot region ofthe laser beam. The laser beam intensity includes two levels, i.e., highand low levels.

A low-level laser beam is radiated, and a medium temperature isincreased beyond T_(comp).1. Thus, the medium type is shifted from Ptype to A type. Although the directions of the RE and TM spins of thefirst layer are left unchanged, the relationship between their strengthsis reversed. As a result, the direction of magnetization of the firstlayer is reversed, as shown in Conditions 2_(La) and 2_(Lb) in FIG. 16.

Therefore the temperature reaches T_(L). Thus the following relation isestablished: ##EQU5## Even if Hb ↑ is present, Condition 2_(La)transitions to Condition 3_(L) in FIG. 16. Meanwhile, since Condition2_(Lb) remains the same regardless of Hb ↑, it becomes the sameCondition 3_(L).

In this state, when the bit falls outside the spot region of the laserbeam, the medium temperature begins to be decreased. When the mediumtemperature is decreased below T_(comp).1, the medium type is restoredfrom A type to original P type. The relationship between the strengthsof the RE and TM spins of the first layer is reversed ( → ).

As a result, the direction of magnetization of the first layer isreversed to the "A direction" ↑, as shown in Condition 4_(L) in FIG. 16.

Condition 4_(L) is held even when the medium temperature is decreased tothe room temperature. As a result, a bit in the "A direction" is formedin the first layer.

A high-temperature cycle will be described below with reference to FIG.17.

When a high-level laser beam is radiated, the medium temperature isincreased to the low temperature T_(L) via T_(comp).1. As a result, thesame Condition 2_(H) as Condition 3_(L) is established.

Under radiation of the high-level laser beam, the medium temperature isfurther increased. When the medium temperature exceeds T_(comp).2 of thesecond layer, the medium type is shifted from A type to P type. Althoughthe directions of the RE and TM spins of the second layer remain thesame, the relationship between their strengths is reversed ( → ). Forthis reason, the direction of magnetization of the second layer isreversed, i.e., the "non-A-directed" magnetization appears (Condition3_(H)).

However, since H_(C2) is still large at this temperature, themagnetization of the second layer will not be reversed by ↑ Hb. When thetemperature is further increased and reaches T_(H), the coercivity ofthe first and second layers are decreased since T_(H) is near the Curietemperature. As a result, the medium satisfies one of the followingFormulas (1) to (3): ##EQU6## For this reason, the directions ofmagnetization of the two layers are reversed at almost the same time,and follow the direction of Hb. This condition corresponds to Condition4_(H).

When the bit falls outside the spot region of the laser beam in thiscondition, the medium temperature begins to fall. When the mediumtemperature is decreased below T_(comp).2, the medium type is shiftedfrom P type to A type. Although the directions of the RE and TM spinsare left unchanged, the relationship between their strengths is reversed( → ). As a result, the direction of magnetization of the second layeris reversed from to the "non-A direction" (Condition 5_(H)).

When the medium temperature is further decreased from the temperature inCondition 5_(H) below T_(comp).1, the medium type is restored from Atype to original P type. The relationship between the strengths of theRE and TM spins of the first layer is then reversed ( → ). As a result,the direction of magnetization of the first layer is reversed to the"non-A direction" (Condition 6_(H)).

The medium temperature is then decreased from the temperature inCondition 6_(H) to the room temperature. Since H_(C1) at the roomtemperature is sufficiently large (Formula 15-3), the magnetization ofthe first layer will not be reversed by ↑ Hb, and Condition 6_(H) ismaintained. ##EQU7## In this manner, a bit in the "non-A direction" isformed in the first layer.

The principle of the over-write operation will be described below usinga specific medium No. 2 belonging to a recording medium of Class 2 (Ptype, Quadrant I, and Type 2) shown in Table 1.

The medium No. 2 has a relation given by Formula 16:

    T.sub.R <T.sub.C1 ≈T.sub.L <T.sub.comp.2 <T.sub.C2 ≈T.sub.H

FIG. 18 shows this relation.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field Hini. without reversing that of thefirst layer at the room temperature T_(R) is given by Formula 17. Thismedium No. 2 satisfies Formula 17. ##EQU8##

In this case, a condition for Hini. is given by Formula 20. When Hini.disappears, the reversed magnetization of the second layer is influencedby that of the first layer due to the exchange coupling force.Conditions for holding the magnetization of the second layer withoutre-reversing it are given by Formulas 18 and 19. The medium No. 2satisfies Formulas 18 and 19. ##EQU9##

Magnetization of the second layer of the recording medium whichsatisfies Conditions given by Formulas 17 to 19 at the room temperatureis aligned in, e.g., the "A direction" ( ) by Hini. satisfying Formula20 immediately before recording. In this case, the first layer is leftin a recorded state, as shown in FIG. 19.

This Condition 1 is maintained immediately before recording. In thiscase, the bias field (Hb) is applied in a direction of ↑.

A low-temperature cycle will be described below with reference to FIG.19.

A low-level laser beam is radiated to increase the medium temperature toT_(L). Since T_(L) is almost equal to the Curie temperature T_(C1) ofthe first layer, its magnetization disappears, and Condition 2_(L) isestablished.

In Condition 2_(L), when the bit falls outside the spot region of thelaser beam, the medium temperature begins to be decreased. When themedium temperature is decreased slightly below T_(C1), RE and TM spins () of the second layer influence those of the first layer due to theexchange coupling force. That is, a force acts to respectively align REspins (↑) and TM spins ( ). As a result, magnetization , i.e., appearsin the first layer, thus setting Condition 3_(L).

Condition 3_(L) is left unchanged even when the medium temperature isfurther decreased. As a result, a bit in the "A direction" is formed inthe first layer.

A high-temperature cycle will be described below with reference to FIG.20.

When a high-level laser beam is radiated and increases the mediumtemperature to T_(L), since T_(L) is almost equal to the Curietemperature T_(C1) of the first layer, its magnetization disappears, andCondition 2_(H) is set.

When the medium temperature slightly exceeds T_(comp).2 of the secondlayer, the relationship between the strengths of RE and TM spins arereversed ( → ) although their directions are left unchanged. For thisreason, the magnetization of the entire alloy is reversed to the "non-Adirection" , and Condition 3_(H) is established. However, since H_(C2)is still large at this temperature, the magnetization of the secondlayer will not be reversed by ↑ Hb. When the temperature is furtherincreased and reaches T_(H), the temperature of the second layer becomesalmost equal to the Curie temperature T_(C2), and its magnetizationdisappears, thus setting Condition 4_(H).

When the bit falls outside the spot region of the laser beam inCondition 4_(H), the medium temperature begins to fall. When the mediumtemperature is decreased slightly below T_(C2), magnetization appears inthe second layer. In this case, magnetization ( ) appears due to ↑ Hb.However, since the temperature is still higher than T_(C1), nomagnetization appears in the first layer. This condition corresponds toCondition 5_(H).

When the medium temperature is further decreased and becomes equal to orlower than T_(comp).2, the relationship between the strengths of RE andTM spins is reversed ( → ) although their directions are left unchanged.As a result, the magnetization of the entire alloy is reversed from tothe "non-A direction" , and Condition 6_(H) is set.

In Condition 6_(H), since the medium temperature is higher than T_(C1),magnetization of the first layer does not appear. In addition, sinceH_(C2) at this temperature is large, the magnetization of the secondlayer will not be reversed by ↑ Hb.

When the temperature is further decreased slightly below T_(C1),magnetization appears in the first layer. In this case, the exchangecoupling force from the second layer acts to respectively align the REspins (↓) and TM spins ( ). For this reason, magnetization , i.e.,appears in the first layer. This condition corresponds to Condition7_(H).

The medium temperature is then decreased from the temperature inCondition 7_(H) to the room temperature. Since H_(C1) at the roomtemperature is sufficiently large, the magnetization of the first layerwill not be reversed by ↑ Hb, and Condition 7_(H) can be maintained.Thus, formation of a bit in the "non-A direction" is completed.

The principle of the over-write operation will be described in detailbelow using a specific medium No. 3 belonging to a recording medium ofClass 3 (P type, Quadrant I, Type 3) shown in Table 1.

This medium No. 3 has a relation given by Formula 21:

    T.sub.R <T.sub.comp.1 <T.sub.C1 ≈T.sub.L <T.sub.C2 ≈T.sub.H

FIG. 21 shows this relation.

A condition for reversing only the magnetization of the second layer bythe initial field Hini. without reversing that of the first layer at theroom temperature T_(R) is given by Formula 22. This medium No. 3satisfies Formula 22. ##EQU10##

In this case, a condition for Hini. is represented by Formula 25. WhenHini. disappears, the reversed magnetization of the second layer isinfluenced by that of the first layer due to the exchange couplingforce. Conditions for maintaining the magnetization of the second layerwithout re-reversing it are given by Formulas 23 and 24. This medium No.3 satisfies Formula 23 and 24. ##EQU11##

Magnetization of the second layer of the recording medium whichsatisfies the conditions given by Formulas 22 to 24 at the roomtemperature is aligned in, e.g., the "A direction" ( ) by Hini.satisfying a condition given by Formula 25 immediately before recording.In this case, the first layer is left in the recorded state, as shown inCondition 1 in FIG. 22.

This Condition 1 is held immediately before recording. In this case, thebias field (Hb) is applied in a direction of ↓. A low-temperature cyclewill be described below with reference to FIG. 22.

A low-level laser beam is radiated to increase the medium temperature toT_(L). Since T_(L) is almost equal to the Curie temperature T_(C1) ofthe first layer, its magnetization disappears. However, since H_(C2) ofthe second layer is still large at this temperature, its magnetizationwill not be reversed by ↓ Hb, and Condition 2_(L) is established.

When the bit falls outside the spot region of the laser beam inCondition 2_(L), the medium temperature begins to fall. When the mediumtemperature is decreased slightly below T_(C1), the RE and TM spins ( )of the second layer influence those of the first layer due to theexchange coupling force. More specifically, a force acts to respectivelyalign RE spins (↑) and TM spins ( ). As a result, magnetization , i.e.,appears in the first layer. In this case, since the temperature is equalto or higher than T_(comp).1, the TM spin becomes larger, and Condition3_(L) is established.

When the medium temperature is decreased below T_(comp).1, therelationship between the strengths of the RE and TM spins of the firstlayer is reversed like in the high-temperature cycle ( → ). As a result,the magnetization of the first layer overcomes ↓ Hb and is aligned in ,thus setting Condition 4_(L).

This Condition 4_(L) is maintained even when the medium temperature isdecreased to the room temperature. As a result, a bit in the "Adirection" is formed.

A high-temperature cycle will be described below with reference to FIG.23.

When a high-level laser beam is radiated and increases the mediumtemperature to T_(L), since T_(L) is almost equal to the Curietemperature T_(C1) of the first layer, its magnetization disappears, andCondition 2_(H) is established.

When the medium temperature reaches T_(H), since T_(H) is almost equalto T_(C2) of the second layer, its magnetization also disappears, andCondition 3_(H) is established.

When the bit falls outside the spot region of the laser beam inCondition 3_(H), the medium temperature begins to fall. When the mediumtemperature is decreased slightly below T_(C2), magnetization appears inthe second layer. In this case, magnetization ( ) appears due to ↓ Hb.However, since the temperature is still higher than T_(C1), nomagnetization appears in the first layer. This condition corresponds toCondition 4_(H).

When the medium temperature is further decreased slightly below T_(C1),magnetization also appears in the first layer. In this case, themagnetization of the second layer influences that of the first layer dueto the exchange coupling force. As a result, a force acts torespectively align RE spins (↓) and TM spins ( ). In this case, sincethe medium temperature is equal to or higher than T_(comp).1, the TMspins are larger than RE spins ( ). As a result, magnetization ( )appears in the second layer, and Condition 5_(H) is established.

When the medium temperature is further decreased from the temperature inCondition 5_(H) and becomes equal to or lower than T_(comp).1, therelationship between the strengths of the TM and RE spins of the firstlayer is reversed ( → ). For this reason, the direction of magnetizationof the first layer is reversed to the "non-A direction" , and Condition6_(H) is established.

The medium temperature is then decreased from the temperature inCondition 6_(H) to the room temperature. Since H_(C1) at the roomtemperature is sufficiently large, the magnetization of the first layeris stably held.

Thus, a bit in the "non-A direction" is formed.

The principle of the over-write operation will be described in detailbelow using a specific medium No. 4 belonging to a recording medium ofClass 4 (P type, Quadrant I, Type 4) shown in Table 1.

This medium No. 4 has a relation given by Formula 26:

    T.sub.R <T.sub.L <T.sub.H <T.sub.C1 <T.sub.C2

For the sake of simplicity, T_(H) <T_(C1) <T_(C2) in the followingdescription. FIG. 24 shows this relation.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field Hini. without reversing that of thefirst layer at the room temperature T_(R) is represented by Formula 27.The medium No. 4 satisfies Formula 27. ##EQU12##

In this case, a condition for Hini. is represented by Formula 30. WhenHini. disappears, the directions of magnetization of the first andsecond layers are influenced by the exchange coupling force. Conditionsfor maintaining the directions of magnetization of the first and secondlayers without reversing them are given by Formulas 28 and 29. Thismedium No. 4 satisfies Formulas 28 and 29. ##EQU13##

The magnetization of the second layer of the recording medium whichsatisfies the conditions given by Formulas 27 to 29 at the roomtemperature is aligned in, e.g., the "A direction" ( ) by Hini. whichsatisfies Formula 30 immediately before recording: ##EQU14## At thistime, the first layer is left in the recorded state shown in Condition1a or 1b in FIG. 25 Condition 1a or 1b is maintained immediately beforerecording. The bias field Hb is assumed to be applied in the "non-Adirection" ↓.

When the medium has a disk shape, a condition for inhibitingmagnetization of a recorded bit (in particular, a bit in Condition 1b inwhich the direction of magnetization of the first layer is opposite tothe direction of Hb) from being reversed by Hb when it approaches an Hbapply means is represented by Formula 30-2: ##EQU15## The disk mediummust satisfy this formula at the room temperature. A condition forinhibiting the initialized second layer from being reversed by Hb whenit approaches the Hb apply means is represented by Formula 30-3:##EQU16## In other words, conditions for determining Hb are Formulas30-2 and 30-3.

A bit in Condition 1a or 1b then reaches the spot region of the laserbeam.

A low-temperature cycle will be described below with reference to FIG.25.

A low-level laser beam is radiated, and the medium temperature isincreased to T_(L). Thus, a condition which can satisfy the followingrelation is established, and Condition 1a transitions to Condition 2_(L): ##EQU17## On the other hand, since Condition 1b is left unchanged, itbecomes the same Condition 2_(L).

In Condition 2_(L), when the bit falls outside the spot region of thelaser beam, the medium temperature begins to be decreased. Even when themedium temperature is decreased to the room temperature, Condition 2_(L)is maintained since its H_(C1) at the room temperature is sufficientlylarge (see Formula 30-4). ##EQU18##

As a result, a bit in the "A direction" is formed in the first layer.

A high-temperature cycle will be described below with reference to FIG.26.

When a high-level laser beam is radiated, the medium temperature isincreased to the low temperature T_(L). As a result, Condition 2_(H)equal to Condition 2_(L) in the low-temperature cycle is established.

When the medium temperature is further increased up to T_(H), thecoercivity is decreased since T_(H) approaches the Curie temperatures ofthe first and second layers. As a result, the medium satisfies one ofthe following Formulas (1) to (3): ##EQU19## For this reason, the directtwo layers are almost simultaneously reversed to follow the direction ofHb. This condition corresponds to Condition 3_(H).

When the bit falls outside the spot region of the laser beam inCondition 3_(H), the medium temperature is begins to be decreased. Themedium temperature is then decreased to the room temperature. However,Condition 3_(H) is left unchanged.

Thus, a bit in the "non-A direction" is formed in the first layer.

The principle of the over-write operation will be described in detailbelow using a medium No. 5 belonging to a recording medium of Class 5 (Atype, Quadrant II, Type 3) shown in Table 1.

This medium No. 5 has a relation given by Formula 31:

    T.sub.R <T.sub.comp.1 <T.sub.L <T.sub.H ≲T.sub.C1 ≲T.sub.C2

For the sake of simplicity, T_(H) <T_(C1) <T_(C2) in the folIowingdescription. FIG. 27 shows this relation.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field Hini. without reversing that of thefirst layer at the room temperature T_(R) is represented by Formula 32.This medium No. 5 satisfies Formula 32. ##EQU20##

In this case, a condition for Hini. is represented by Formula 35. WhenHini. disappears, the directions of magnetization of the first andsecond layers are influenced by each other due to the interface wallenergy. Conditions for maintaining the directions of magnetization ofthe first and second layers without reversing them are represented byFormulas 33 and 34. This medium No. 5 satisfies Formulas 33 and 34.##EQU21##

Magnetization of the second layer of the recording medium whichsatisfies the conditions given by Formulas 32 to 34 at the roomtemperature is aligned in, e.g., the "A direction" ( ) by Hini. whichsatisfies Formula 35 immediately before recording: ##EQU22## At thistime, the first layer is left in the recorded state shown in Condition1_(a) or 1_(b) in FIG. 28. This Condition 1a or 1b is maintainedimmediately before recording.

The bias field Hb is assumed to be applied in the "non-A direction" ↓.

When the medium has a disk shape, a condition for inhibitingmagnetization of a previously recorded bit (in particular, a bit inCondition 1a in which the direction of magnetization of the first layeris opposite to the direction of Hb) from being reversed by Hb when itapproaches an Hb apply means is represented by Formula 35-2: ##EQU23##The disk medium must satisfy this formula at the room temperature. Acondition for inhibiting the initialized second layer from beingreversed by Hb when it approaches the Hb apply means is represented byFormula 35-3: ##EQU24## In other words, conditions for determining Hbare Formulas 35-2 and 35-3.

A low-temperature cycle will be described below with reference to FIG.28.

A low-level laser beam is radiated, and the medium temperature isincreased beyond T_(comp).1. The medium type is changed from A type to Ptype. The relationship between the strengths of the RE and TM spins ofthe first layer is reversed although their directions are leftunchanged. For this reason, the direction of magnetization of the firstlayer is reversed, so that Condition 1_(a) becomes Condition 2_(La), andCondition 1_(b) becomes Condition 2_(Lb).

When the medium temperature is further increased from this state up toT_(L), the following formula is satisfied: ##EQU25## Thus, Condition2_(La) transitions to Condition 3_(L). Meanwhile, since Condition 2_(Lb)is left unchanged, it becomes the same Condition 3_(L).

When the bit falls outside the spot region of the laser beam in thiscondition, the medium temperature begins to be decreased. When themedium temperature is decreased below T_(comp).1, the medium type isrestored from P type to original A type. The relationship between thestrengths of RE and TM spins of the first layer is reversed ( → ). As aresult, the direction of magnetization of the first layer is reversed tothe "non-A direction" . This condition corresponds to Condition 4_(L).

The medium temperature is then decreased to the room temperature, andCondition 4_(L) is maintained.

As a result, a bit in the "non-A direction" is formed in the firstlayer.

A high-temperature cycle will be described below with reference to FIG.29.

When a high-level laser beam is radiated, the medium temperature isincreased to the low temperature T₁ via T_(comp).1.As a result,Condition 2_(H) equal to Condition 3_(L) is established.

When the reaches T_(H), the coersivities of the two layers are decreasedsince T_(H) is near the Curie temperatures of the first and secondlayers. As a result, the medium satisfies one of the following Formulas(1) to (3): ##EQU26## For this reason, the directions of magnetizationof the two layers are almost simultaneously reversed to follow thedirection of Hb. This condition corresponds to Condition 3_(H).

When the bit falls outside the spot region of the laser beam inCondition 3_(H), the medium temperature begins to be decreased. When themedium temperature is decreased below T_(comp).1, the medium type isrestored from P type to original A type. The relationship between thestrengths of TM and RE spins of the first layer is reversed ( → ). Forthis reason, the direction of magnetization of the first layer isreversed to the "A direction" (Condition 4_(H)). The medium temperatureis decreased from the temperature in Condition 4_(H) to the roomtemperature. H_(C1) at the room temperature is sufficiently large, andFormula 35-4 is satisfied: ##EQU27## Therefore, the magnetization of thefirst layer is stably maintained in Condition 4_(H).

Thus, a bit in the "A direction" is formed in the first layer.

The principle of the over-write operation will be described in detailbelow using a specific medium No. 6 belonging to a recording medium ofClass 6 (A type, Quadrant II, Type 4) shown in Table 1.

This medium No. 6 has a relation given by Formula 36:

    T.sub.R <T.sub.C1 ≈T.sub.L <T.sub.C2 ≈T.sub.H

FIG. 30 shows this relation.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field Hini. without reversing that of thefirst layer at the room temperature T_(R) is represented by Formula 37.This medium No. 6 satisfies Formula 37. ##EQU28##

A condition for Hini. at this time is given by Formula 40. When Hini.disappears, the reversed magnetization of the second layer is influencedby that of the first layer due to the exchange coupling force.Conditions for holding the magnetization of the second layer withoutre-reversing it are given by Formulas 38 and 39. This medium No. 6satisfies Formulas 38 and 39. ##EQU29##

The magnetization of the second layer of the recording medium whichsatisfies the conditions given by Formulas 37 to 39 at the roomtemperature is aligned in, e.g. the "A direction" ( ) by Hini. whichsatisfies the condition given by Formula 40 immediately beforerecording. At this time, the first layer is kept in the recorded state,as shown in Condition 1 in FIG. 31. Condition 1 is maintainedimmediately before recording. In this case, the bias field (Hb) isapplied in a direction of ↓.

A low-temperature cycle will be described below with reference to FIG.31.

A low-level laser beam is radiated to increase the medium temperature toT_(L). Since T_(L) is almost equal the Curie temperature T_(C1) of thefirst layer, its magnetization disappears. In this condition, sinceH_(C2) is sufficiently large, the magnetization of the second layer willnot be reversed by ↓ Hb. This condition corresponds to Condition 2_(L).

When the bit falls outside the spot region of the laser beam inCondition 2_(L), the medium temperature begins to be decreased. When themedium temperature is decreased slightly below T_(C1), the RE and TMspins ( ) of the second layer influence those of the first layer due tothe exchange coupling force. The exchange coupling force acts torespectively align the RE spins (↓) and TM spins ( ). As a result,magnetization , i.e., appears in the first layer. This conditioncorresponds to Condition 3_(L).

Condition 3_(L) is maintained even when the medium temperature isdecreased to the room temperature. As a result, a bit in the "non-Adirection" is formed.

A high-temperature cycle will be described below with reference to FIG.32.

When a high-level laser beam is radiated and increases the mediumtemperature to T_(L), since T_(L) is almost equal to the Curietemperature T_(C1) of the first layer, its magnetization disappears,i.e., Condition 2_(H) is established.

When the medium temperature is further increased up to T_(H), since thetemperature T_(H) of the second layer is almost equal to T_(C2), itsmagnetization also disappears. This condition corresponds to Condition3_(H).

When the bit falls outside the spot region of the laser beam inCondition 3_(H), the medium temperature begins to be decreased. When themedium temperature is decreased slightly below T_(C2), magnetizationappears in the second layer. In this case, magnetization ( ) appears dueto ↓ Hb. However, since the temperature is higher than T_(C1), nomagnetization appears in the first layer. This condition corresponds toCondition 4_(H).

When the medium temperature is further decreased slightly below T_(C1),magnetization appears in the first layer. At this time, the exchangecoupling force from the second layer acts to respectively align RE spins(↑) and TM spins ( ). As a result, magnetization , i.e., overcomes ↓ Hb,and appears in the first layer. This condition corresponds to Condition5_(H).

The medium temperature is then decreased from the temperature inCondition 5_(H) to the room temperature. Since H_(C1) at the roomtemperature is sufficiently large, the magnetization of the first layeris stably maintained. Thus, a bit in the "A direction" is formed.

The principle of the over-write operation will be described in detailbelow using a medium No. 7 belonging to a recording medium of Class 7 (Ptype, Quadrant III, Type 4) shown in Table 1.

This medium No. 7 has a relation given by Formula 41:

    T.sub.R <T.sub.L <T.sub.H ≲T.sub.C1 ≲T.sub.C2

For the sake of simplicity, T_(H) <T_(C1) <T_(C2) in the followingdescription. FIG. 33 shows this relation.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field Hini. without reversing that of thefirst layer at the room temperature T_(R) is represented by Formula 42.This medium No. 7 satisfies Formula 42. ##EQU30##

A condition for Hini. at this time is given by Formula 45. When Hini.disappears, the directions of magnetization of the first and secondlayers influence each other due to the interface wall energy. Conditionsfor maintaining the directions of magnetization of the first and secondlayers without reversing them are represented by Formulas 43 and 44.This medium No. 7 satisfies Formulas 43 and 44. ##EQU31##

The magnetization of the second layer of the recording medium whichsatisfies conditions given by Formulas 42 to 44 at the room temperatureis aligned in, e.g., the "A direction" ( ) by Hini. which satisfies thecondition given by Formula 45 immediately before recording: ##EQU32## Atthis time, the first layer is left in Condition 1_(a) or 1_(b) shown inFIG. 34.

The bias field Hb is assumed to be applied in the "non-A direction" ↓.

When the medium has a disk shape, a condition for inhibitingmagnetization of a recorded bit (in particular, a bit in Condition 1_(b)in which the direction of magnetization of the first layer is oppositeto the direction of Hb) from being reversed by Hb when it approaches anHb apply means is represented by Formula 44-2: ##EQU33## The disk mediummust satisfy this formula at the room temperature. A condition forinhibiting the initialized second layer from being reversed by Hb whenit approaches the Hb apply means is represented by Formula 45-3:##EQU34## In other words, conditions for determining Hb are Formulas45-2 and 45-3.

A low-temperature cycle will be described below with reference to FIG.34.

A low-level laser beam is radiated, and the medium temperature isincreased to T_(L). Thus, the following condition is satisfied:##EQU35## Thus, Condition 1a transitions to Condition 2_(L). Meanwhile,Condition 1b is maintained, and becomes Condition 2_(r).

When the bit falls outside the spot region of the laser beam incondition 2_(L), the medium temperature begins to be decreased. SinceH_(cl) is sufficiently large at the room temperature (see Formula 45-2,Condition 2_(L) can be maintained at the room temperature.

As a result, a bit in the "A direction" is formed in the first layer.

A high-temperature cycle will be described below with reference to FIG.35.

A high-level laser beam is radiated, and the medium temperature isincreased to the low temperature T_(L). As a result, Condition 2_(H)equal to Condition 2_(L) is established.

The temperature thereafter reaches T_(H). Since T_(H) approaches theCurie temperatures of the first and second layers, the medium satisfiesone of the following Formulas (1) to (3): ##EQU36## For this reason, thedirections of magnetization of the two layers are almost simultaneouslyreversed to follow the direction of ↓ Hb. This condition corresponds toCondition 3_(H).

When the bit falls outside the spot region of the laser beam inCondition 3_(H), the medium temperature begins to be decreased.

The medium temperature is then returned to the room temperature.However, Condition 3_(H) is left unchanged.

As a result, a bit in the "non-A direction" is formed in the firstlayer.

The principle of the over-write operation will be described in detailbelow using a medium No. 8 belonging to a recording medium of Class 8 (Atype, Quadrant IV, Type 2) shown in Table 1.

This medium No. 8 has a relation given by Formula 46:

    R.sub.R <T.sub.L <T.sub.H ≲T.sub.C1 ≲T.sub.C2

For the sake of simplicity, T_(H) <T_(C1) <T_(C2) in the followingdescription. T_(comp).2 may be lower than, or equal to, or higher thanT_(L) or T_(C1). However, for the sake of simplicity, T_(L) <T_(comp).2<T_(C1) in the following description. FIG. 36 shows this relation.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field room temperature T_(R) is representedby Formula 47. This medium No. 8 satisfies Formula 47 at the roomtemperature. ##EQU37##

A condition for Hini. at this time is given by Formula 50. When Hini.disappears, the directions of magnetization of the first and secondlayers influence each other due to the interface wall energy. Conditionsfor maintaining the directions of magnetization of the first and secondlayers without reversing them are represented by Formulas 48 and 49.This medium No. 8 satisfies Formulas 48 and 49. ##EQU38##

The magnetization of the second layer of the recording medium whichsatisfies Formulas 47 to 49 at the room temperature is aligned in, e.g.,the "A direction" ( ) by Hini. which satisfies the condition given byFormula 50 immediately before recording: ##EQU39## At this time, thefirst layer is left in the recorded state, as shown in Condition 1_(a)or 1_(b) in FIG. 37.

The bias field Hb is assumed to be applied in the "A direction" ↑.

When the medium has a disk shape, a condition for inhibitingmagnetization of a bit recorded just one revolution before (inparticular, a bit in Condition 1b in which the direction ofmagnetization of the first layer is opposite to the direction of Hb)from being reversed by Hb is represented by Formula 50-2: ##EQU40## Thedisk medium must satisfy this formula at the room temperature. In otherwords, a condition for determining Hb is given by Formula 50-2.

A low-temperature cycle will be described below with reference to FIG.37.

A low-level laser beam is radiated, and the medium temperature isincreased to T_(L). Thus, the following condition is satisfied:##EQU41## Thus, Condition 1a transitions to Condition 2_(L). Meanwhile,Condition 1b is maintained, and becomes Condition 2_(L).

When the bit falls outside the spot region of the laser beam inCondition 2_(L), the medium temperature begins to be decreased. Evenwhen the medium temperature is decreased to the room temperature,Condition 2_(L) is maintained since H_(C1) is sufficiently large (seeFormula 50-2).

As a result, a bit in the "non-A direction" is formed in the firstlayer.

A high-temperature cycle will be described below with reference to FIG.38.

A high-level laser beam is radiated, and the medium temperature isincreased to the low temperature T_(L). As a result, Condition 2_(H)equal to Condition 2_(L) in the low-temperature cycle is established.

Under radiation of the high-level laser beam, the medium temperature isfurther increased. When the medium temperature exceeds T_(comp). 2, themedium type is shifted from A type to P type. The relationship betweenthe strengths of RE spin (↑) and TM spin ( ) of the second layer isreversed ( → ) while their directions are left unchanged. As a result,the direction of magnetization of the second layer is reversed to the"non-A direction" . This condition corresponds to Condition 3_(H).

However, since H_(C2) is still large at this temperature, themagnetization the second layer will not be reversed by ↑ Hb.

The medium temperature is further increased up to T_(H). Since themedium temperature is near the Curie temperatures of the first andsecond layers, the coercivities of the two layers are decreased. As aresult, the medium satisfies one of following Formulas (1) to (3):##EQU42## For this reason, the directions of magnetization of the twolayers are almost simultaneously reversed to follow the direction of↑Hb. This condition corresponds to Condition 4_(H).

When the bit falls outside the spot region of the laser beam inCondition 4_(H), the medium temperature begins to be decreased. When themedium temperature is decreased below T_(comp). 2, the medium type isrestored from P type to original A type. The relationship between thestrengths of RE spin (↓) and TM spin ( ) is reversed ( → ) while theirdirections are left unchanged. As a result, the direction ofmagnetization of the second layer is reversed to the "non-A direction" .In this condition, since H_(C2) has already been considerably large, themagnetization of the second layer will not be reversed by ↑ Hb. Thiscondition corresponds to Condition 5_(H).

The medium temperature is then decreased from the temperature inCondition 5_(H) to the room temperature. However, Condition 5_(H) isleft unchanged.

In this manner, a bit in the "A direction" is formed in the first layer.

The principle of the over-write operation will be described in detailbelow using a specific medium No. 9 belonging to a recording medium ofClass 9 (A type, Quadrant IV, Type 4) shown in Table 1.

This medium No. 9 has a relation given by Formula 51:

    T.sub.R <T.sub.C1 ≈T.sub.L <T.sub.C2 ≈T.sub.H

This relation is shown in FIG. 39.

A condition for reversing only the direction of magnetization of thesecond layer by the initial field Hini. without reversing that of thefirst layer at the room temperature T_(R) is represented by Formula 52.This medium No. 9 satisfies Formula 52. ##EQU43##

A condition for Hini. at this time is given by Formula 55. When Hini.disappears, the reversed magnetization of the second layer is influencedby that of the first layer due to the exchange coupling force.Conditions for holding the magnetization of the second layer withoutre-reversing it are given by Formulas 53 and 54. This medium No. 9satisfies Formulas 53 and 54. ##EQU44##

The magnetization of the second layer of the recording medium whichsatisfies the conditions given by Formulas 52 to 54 at the roomtemperature is aligned in, e.g. the "A direction" ( ) by Hini. whichsatisfies the condition given by Formula 55 immediately beforerecording. At this time, the first layer is in Condition 1 in FIG. 40.

In this case, the bias field (Hb) is applied in a direction of ↓.

A low-temperature cycle will be described below with reference to FIG.40.

A low-level laser beam is radiated to increase the medium temperature toT_(L). Since T_(L) is almost equal to the Curie temperature T_(C1) ofthe first layer, its magnetization disappears. In this condition, sinceH_(C2) is sufficiently large, the magnetization of the second layer willnot be reversed by ↓ Hb. This condition corresponds to Condition 2_(L).

When the bit falls outside the spot region of the laser beam inCondition 2_(L), the medium temperature begins to be decreased. When themedium temperature is decreased slightly below T_(C1), RE and TM spins () of the second layer influence those of the first layer due to theexchange coupling force. The exchange coupling force acts torespectively align the RE spins (↑) and TM spins ). As a result,magnetization , i.e. appears in the first layer. This conditioncorresponds to Condition 3_(L).

Condition 3_(L) is maintained even when the medium temperature isdecreased to the room temperature. As a result, a bit in the "non-Adirection" is formed.

A high-temperature cycle will be described below with reference to FIG.41.

When a high-level laser beam is radiated and increases the mediumtemperature to T_(L), since T_(L) is almost equal to the Curietemperature T_(C1) of the first layer, its magnetization disappears, andCondition 2_(H) is established.

When the medium temperature is further increased up to T_(H), since thetemperature T_(H) of the second layer is almost equal to T_(C2),magnetization of the second layer also disappears. This conditioncorresponds to Condition 3_(H).

When the bit falls outside the spot region of the laser beam inCondition 3_(H), the medium temperature begins to be decreased. When themedium temperature is decreased slightly below T_(C2), magnetizationappears in the second layer. In this case, magnetization ( ) appears dueto ↓ Hb. However, since the temperature is higher than T_(C1), nomagnetization appears in the first layer. This condition corresponds toCondition 4_(H).

When the medium temperature is further decreased slightly below T_(C1),magnetization appears in the first layer. At this time, the exchangecoupling force from the second layer ( ) acts to respectively align REspins (↓) and TM spins ( ). As a result, magnetization , i.e., overcomes↓ Hb, and appears in the first layer. This condition corresponds toCondition 5_(H).

The medium temperature is then decreased from the temperature inCondition 5_(H) to the room temperature. Since H_(C1) at the roomtemperature is sufficiently large, the magnetization of the first layeris stably maintained. Thus, a bit in the "A direction" is formed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for explaining bit formation of a magnetoopticalrecording medium,

FIG. 2 is a view for explaining reproduction of information of themagnetooptical recording medium,

FIGS. 3 and 4 are charts showing output waveforms of a laser beam wheninformation is recorded on the magnetooptical recording medium,

FIG. 5 is a view showing a structure of the magnetooptical recordingmedium,

FIG. 6 is a graph showing the relationship between coercivity andtemperature of a magnetooptical recording medium according to the firstcategory of the present invention,

FIG. 7 is a schematic view of the magnetooptical recording medium shownin FIG. 5,

FIG. 8 is a diagram showing a change in magnetization when a high-levellaser beam is radiated on the magnetooptical recording medium shown inFIG. 7 to record information,

FIG. 9 is a diagram showing a change in magnetization when a low-levellaser beam is radiated on the magnetooptical recording medium shown inFIG. 7 to record information,

FIG. 10 is a diagram for explaining an over-write operation of themagnetooptical recording medium according to the present invention,

FIG. 11 is a diagram showing a change in magnetization when a high-levellaser beam is radiated on a magnetooptical recording medium according tothe second category of the present invention,

FIG. 12 is a diagram showing a change in magnetization when a low-levellaser beam is radiated on the magnetooptical recording medium accordingto the second category of the present invention,

FIG. 13 and FIGS. 14A, 14B, 14C, and 14D are views showingcharacteristics of different types of magnetooptical recording mediaaccording to the present invention,

FIGS. 15, 18, 21, 24, 27, 30, 33, 36, and 39 are graphs showing therelationships between coercivities and temperatures of magnetoopticalrecording media according to the present invention,

FIGS. 16, 19, 22, 25, 28, 31, 34, 37, and 40 are diagrams showinglow-temperature cycles of the magnetooptical recording media accordingto the present invention,

FIGS. 17, 20, 23, 26, 29, 32, 35, 38, and 41 are diagrams showinghigh-temperature cycles of the magnetooptical recording media accordingto the present invention,

FIGS. 42A and 42B are sectional views for explaining an embodiment ofthe present invention,

FIG. 42C is an additional explanatory view, and

FIG. 43 is a diagram showing an apparatus according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below in connectionwith preferred embodiments thereof utilizing a Class 8 medium. However,the present invention is not limited to these.

Manufacture of Class 8 Medium

(1) First, a disk-like glass substrate (S) having a thickness of 1.2 mm,and a diameter of 130 mm is prepared, and a grooved layer (U) having athickness of about 100 μm is formed thereon. The grooved layer (U) isformed of an ultraviolet-curing resin, and a spiral groove for forming aseparation zone of the present invention is formed on the layer. Thegroove has a depth of 700 Å, and a width of 0.4 μm, as shown in FIG.42B.

(2) An RF magnetron-sputtering apparatus is used, and the glasssubstrate (S) with the grooved layer is set in a vacuum chamber.

After the interior of the vacuum chamber is temporarily evacuated to5×10⁻⁵ Pa., argon gas is introduced, and sputtering is performed whilemaintaining an Ar gas pressure to be 2×10⁻¹ Pa.

As shown in FIG. 42A, SiN is used as a first target, thus forming a700-Å thick first protection layer 3 on the grooved layer. Subsequently,a Tb₂₀ Fe₇₆ Co₄ alloy is used as a target, thereby forming a first layer1 (recording layer) comprising a Tb₂₀ Fe₇₆ Co₄ perpendicular magneticanisotropy film having a film thickness t₁ =500 Å. Note that the unit ofsuffixes in an alloy composition is atomic %. The same applies to thefollowing description.

Sputtering is performed using a Tb₉ Dy₁₈ Fe₄₅ Co₂₈ alloy as a targetwhile maintaining a vacuum state, thereby forming a second layer 2(reference layer) comprising a Tb₉ Dy₁₈ Fe₄₅ Co₂₈ perpendicular magneticanistotropy film having a film thickness t₂ =1,500 Å.

Finally, a second 700-Å thick SiN protection layer 3 is formed on thesecond layer.

Table 2 below summarizes magnetic characteristics (25° C.) oftwo-layered magnetooptical recording media belonging to Class 8 (A type,Quadrant IV, Type 2) manufactured in this manner.

                  TABLE 2                                                         ______________________________________                                                     First Layer                                                                              Second Layer                                          ______________________________________                                        Composition    Tb.sub.20 Fe.sub.76 Co.sub.4                                                               Tb.sub.9 Dy.sub.18 Fe.sub.45 Co.sub.28            Film Thickness t (Å)                                                                     500          1,500                                             Ms (emu/cc)     35          120                                               Hc (Oe)        12,000       2,000                                             Tc (°C.)                                                                              180          280                                               T.sub.comp. (°C.)                                                                     None         120                                               σ.sub.w (erg/cm.sup.2)                                                                 3.7                                                            ______________________________________                                    

EXAMPLE 1

FIG. 43 is a schematic view showing a structure of an over-write capablemagnetooptical recording apparatus.

Reference numeral 20 denotes an over-write capable magnetoopticalrecording medium; 21, a rotating means for rotating the magnetoopticalrecording medium; 22, an initial field apply means (Hini.=4,000 Oe and"non-A direction" ↓); 23, a laser beam source; 24, a modulation meansfor pulse-modulating a laser beam intensity between high level and lowlevel according to information; and 25, a bias field apply means (Hb=300Oe, and "non-A direction" ↓).

For purposes of the invention, the initial field apply means was removedfrom the apparatus of FIG. 43, and a rod-like permanent magnet forgenerating a 1,000-Oe magnetic field in an "A direction" ↑was mounted asa pre-processing field apply means in place of the bias field applymeans.

A tracking mechanism of the recording apparatus was adjusted so that alaser beam could be radiated along a magnetic layer (multilayeredstructure of first and second layers) located in a groove. A recordingmedium (A type disk) manufactured as earlier described was then set inthe foregoing apparatus and rotated at 1,800 rpm.

In this case, the laser beam intensity was set to be constant, i.e., 10mW on the surface of the magnetic layer, and was not modulated.

In this manner, pre-processing was completed, and the direction ofmagnetization of the first layer in the groove was aligned in the "Adirection" ↑. Since the first layer does not have a compensationtemperature between a medium temperature when it is heated by the laserbeam and a room temperature, the direction of the pre-processing fieldapplied upon radiation of the laser beam is the same as the aligneddirection of magnetization of the first layer at the room temperature.

Since the direction of Hini. is the "non-A direction" ↓, the directionof magnetization of the second layer located in the groove is alsoaligned in the "non-A direction" ↓. However, since this medium is of Atype, when the first layer is aligned in the "A direction" ↑, nomagnetic wall is formed between the first and second layer.

A label indicating that the direction of Hini. should be set in the"non-A direction" ↓ was adhered to the central portion of the mediumprocessed in Example 1. See FIG. 42C.

Note that a different type of indication, such as a "mark" which can bedetected by an electrical, magnetic, or mechanical sensor, may be usedin place of the label. Also, an appropriate indication may be applied toa case containing the medium.

The medium of Example 2 was set in a recording apparatus as shown inFIG. 43. The medium was rotated at 1,800 rpm, and recording wasperformed on a track at a position corresponding to a radius r=30 mm.

A laser beam intensity at high level was set to be 7.0 mW on the surfaceof the recording layer, and an intensity at low level was set to be 3.5mW. Thus, when a high-level laser beam was radiated, the mediumtemperature was increased to a high temperature T_(H) =200° C., and ahigh-temperature process was executed, while when a low-level laser beamwas radiated, the medium temperature was increased to a low temperatureT_(L) =130° C., and a low-temperature process was executed.

A 1-MHz signal wave was used as information, and recording was performedon a track while modulating a laser beam at 1 MHz, and applying Hini.and Hb.

When recorded information was reproduced by another conventionalmagnetooptical recording/reproduction apparatus, a C/N ratio was 55 dB.

A 2-MHz signal wave was used as information, and recording (over-write)and reproduction were performed. As a result, no 1-MHz signal wasobserved at all, and a C/N ratio was 52 dB.

COMPARATIVE EXAMPLE

For the purpose of comparison, substantially the same processing as inExample 1 was performed, except that the pre-processing field waschanged to the "non-A direction" ↓.

Recording/reproduction of this medium was performed in the same manneras explained above As a result, after the over-write operation, a C/Nratio (2-MHz signal) was 50 dB, and a previously recorded 1-MHz signalwhich remained after the over-write operation was observed. Thus, anerase rate of a 1-MHz signal was 40 dB.

As will be appreciated from the preceding description, according to thepresent invention, since processing is performed to eliminate a magneticwall between first and second layers of a recording layer located in aseparation zone, problems such as a decrease in C/N ratio, reproductionof previous information, and a high bit error rate can be solved.

I claim:
 1. Method of pre-processing comprising:(a) a first step of providing a magnetooptical recording medium which is over-write capable by beam intensity modulation, the medium having an at least two-layered structure including a recording layer having a perpendicular magnetic anisotropy and a reference layer having a perpendicular magnetic anisotropy and exchange-coupled to the recording layer, and a direction of magnetization of the reference layer being alignable in a first predetermined direction without changing a direction of magnetization of the recording layer, the medium further having a plurality of tracks in which information is recordable and a separation zone formed between adjacent tracks; and (b) a second step of aligning a direction of magnetization of the recording layer in the separation zone in a second predetermined direction by application of a preprocessing field so as to prevent, when the direction of magnetization of said reference layer is aligned in said first direction, formation of a magnetic wall between the recording layer and the reference layer in the separation zone.
 2. A method according to claim 1, wherein said medium is of parallel type (P type), and said first predetermined direction and said second predetermined direction are the same.
 3. A method according to claim 1, wherein said medium is of anti-parallel type (A-type), and said first predetermined direction and said second predetermined direction are opposite.
 4. A method according to claim 1, wherein said medium is heated during said second step.
 5. A method according to claim 1, further comprising:(c) an additional step of applying to said medium an indication that the direction of magnetization of said recording layer in said separation zone is aligned in said second predetermined direction.
 6. A method according to claim 1, further comprising:(c) an additional step of applying to a case containing said medium an indication that the direction of magnetization of said recording layer in said separation zone is aligned in said second predetermined direction.
 7. A method according to claim 1, further comprising, after said second step:(c) an additional step of aligning the direction of magnetization of said reference layer in said first predetermined direction by application of an initial field.
 8. A magnetooptical recording medium, comprising an at least two-layered structure over-write capable by beam intensity modulation and including a recording layer having a perpendicular magnetic anisotropy and a reference layer having a perpendicular magnetic anisotropy and exchange coupled to the recording layer, with a direction of magnetization of the reference layer being alignable in a first predetermined direction without changing a direction of magnetization of the recording layer, a plurality of tracks in which information is recordable, and a separation zone formed between adjacent tracks, the medium having been subjected to a pre-processing field, with or without heating, so as to align a direction of magnetization of the recording layer in the separation zone in a second predetermined direction such that, when the direction of magnetization of the reference layer is aligned in the first predetermined direction, a magnetic wall between the recording layer and the reference layer is not formed in the separation zone.
 9. A medium according to claim 8, wherein an indication that the direction of magnetization of said recording layer in said separation zone is aligned in said second predetermined direction is applied to said medium itself or to a case containing said medium.
 10. A medium according to claim 9, wherein said first predetermined direction and said second predetermined direction are the same.
 11. A medium according to claim 9, wherein said first predetermined direction and said second predetermined direction are opposite. 